As an optical filter in an image pickup apparatus, an ND (Neutral Density) filter that limits a transmittance may be used. In particular, in order to control a brightness of an image arbitrarily or improve (a decrease of a quality of the image caused by) variation of sharpness of an edge of an out-of-focus image (blurred image), a gradation ND filter that has a transmittance which gradually varies depending on a region in an optical plane is used.
PTL 1 discloses a gradation ND filter which controls a transmittance and a reflectance according to a film thickness of a laminated film for each region. PTL 2 discloses an ND filter which reduces the reflectance by using an antireflection structure.